The instant invention relates to systems and methods for performing material property measurements, and more particularly to systems and methods for identifying material property defects through the use of ultrasonic measuring equipment.
Most, if not all, consumer and industrial products, equipment, and component parts carry with them some form of serial number or product identification code information. One function of this serial number or product identification information is to individually identify particular products or equipment for warranty and maintenance service tracking. Another function, and with particular classes of consumer and industrial products or component parts possibly a primary function, is to serve as a mechanism to track ownership in case of theft or use in crime.
While many products and component parts include separately affixed identification plates or labels including the serial number or product identification code information, particular classes of products and components have this information stamped or engraved in the product or component part itself. Probably two of the most well known consumer products and component parts that utilize this more permanent technique of stamping or engraving product serial number information thereon are firearms and some automotive parts, e.g. engine blocks. As with other products and components parts, these stamped or engraved serial numbers serve to identify and allow the tracking of proper ownership in case of theft, or use in a crime. However, many criminals have recognized that the serial number information may well provide the authorities the very information that would lead to their arrest, and so have engaged in the removal of the serial number information. Often times the criminals simply machine off the stamped or engraved serial number.
Unfortunately, with current technology the tracking of proper ownership of these products and component parts once the serial number has been machined off, absent other evidence, has proven to be extremely difficult for law enforcement officials. Since many criminals routinely engage in the removal of stamped or engraved serial number information from auto parts and fire arms in attempt to conceal their or the rightful owner""s identity, there exists an urgent need for new technology that enables the recovery of this machined-away serial number information.
The ability to extract serial number information from products and components that have been subject to or used in a crime would greatly enhance law enforcement""s ability to apprehend criminals and return property to its rightful owner. The stamping and engraving processes used to place the serial number on the product or component not only results in a visually perceptible impression on the surface of the product or component, but it also introduces a distortion in the microstructure of the material itself well below the surface indentation. As recognized by the inventor of the instant application, the micro-structural distortion is localized to the serial number impression and remains even after the visually perceptible surface indentation has been removed through a machining operation. Unfortunately, since this micro-distortion is localized to the small material volume of the serial number impression, conventional ultrasonic techniques of detecting variations in a material volume are unable to detect and map these distortions.
Measurements of acoustic velocity and attenuation are used in various applications to detect changes in material properties. For example, uniformity in metal microstructure can be sensed by monitoring uniformity in acoustic velocity and attenuation. Such measurements have traditionally been performed using ultrasonic signal propagation paths with lengths measured in tens or hundreds of wavelengths. This is due to the fact that the ability to detect a subtle difference in acoustic velocity or attenuation increases directly with the length of material through which the signal propagates. Such a measurement could be used, for example, to determine uniformity in material processing from one batch of material to the next by comparing differences in measured acoustic properties in representative samples, each being several inches in length.
Such measurements may be performed using instrumentation configured as indicated in FIG. 13. In such a configuration, an ultrasonic pulse is transmitted through a specimen 21 several inches long by placing two ultrasonic transducers 23, 25 on either end of the specimen 21, one 23 to transmit and the other 25 to receive. Typical transducers might be 0.5 inch in diameter, and be design to transmit broadband ultrasonic signals with a 5 MHz center frequency. Ultrasonic wavelengths typically range from 0.1 to 1.0 mm.
An obvious drawback of the measurement technique illustrated in FIG. 13 is the inability to detect localized differences in material properties within the sample volume. That is, the system of FIG. 13 has poor spatial resolution. One technique that does allow the detection of localized variations in material acoustic properties is scanning acoustic microscopy. This technique is similar in approach to that shown in FIG. 13 except that it does not use transmitting 23 and receiving 25 transducers on opposite end of a specimen 21. Instead, a pair of focused transducers 27, 29 are used to propagate a surface wave a short distance over the surface of a specimen 21 as illustrated in FIG. 14.
A surface wave is a special type of wave in a solid material that clings to the surface, as opposed to penetrating through the solid. As illustrated in this FIG. 14, a surface wave can be generated by launching a pulse through water toward the surface at a specific critical angle, slightly greater than the critical angle for total wave reflection. When the pulse hits the surface, it will generate a surface wave pulse within the solid. As the surface wave propagates over the surface, it radiates energy back into the water. By using a pair of focused transducers, one 27 to generate a surface wave pulse at some position and a second 29 to detect the pulse radiated back into the water a short distance away from the generation point, a signal path can be established that includes a small volume of solid material.
The surface wave path L can be adjusted in length from a fraction of a wavelength to a few wavelengths. Acoustic attenuation and velocity in the solid material can be determined by monitoring received signal transit times and amplitudes. By mechanically scanning the transducer pair over the specimen surface, an image can be formed of material properties throughout a thin layer near the specimen surface, with a relatively high spatial resolution. The drawback of the scanning acoustic microscopy measurement depicted in FIG. 14 is that, while measurements can be made with high spatial resolution (e.g. a fraction of a wavelength), measurement sensitivity to variation in material properties suffers because of the relatively short propagation path L in the solid material. As such, small residual material distortions remaining after a serial number has been machined off a component cannot be reliably recovered. A need exists, therefore, for a measurement system and method that has high sensitivity to material properties, as with the measurement system of FIG. 13, yet at the same time has high spatial resolution, as with the measurement system of FIG. 14.
It is therefore an object of the instant invention to overcome these and other problems existing in the art. More specifically, it is an object of the instant invention to provide a system an a method to aid in the recovery of serial number or other identification information from consumer or industrial products and component parts that have had this information machined away. It is a further object of the invention to provide a system and method to detect small, local residual material variations in the product or component originally resulting from the serial number stamping or engraving process. Additionally and more generally, it is an object of the instant invention to provide a system and method for the measurement of material properties in small volumes of the material. It is a still further object of the invention to obtain this measurement with conventional laboratory-grade measurement instruments.
In view of these and other objects of the invention, it is a feature of the invention to provide a system and method to perform highly sensitive measurement of localized ultrasonic attenuation and velocity in small material volumes. It is a further feature of the invention to allow a mapping of the variation of these ultrasonic properties on the surface of the product, component, or other specimen with a high spatial resolution. Additionally, it is a feature of the invention to provide such high spatial resolution of the measurement in material volumes on the order of a wavelength in length.
In view of these objects and features, the system and method of the invention perform high-sensitivity acoustic property measurements in small material volumes. As will be discussed more fully below, in conventional ultrasound measurements a tradeoff is maintained between spatial resolution (the size of the measured material volume) and the measurement sensitivity (the smallest detectable difference in velocity and/or attenuation through the material). In the instant invention, a technique has been devised for circumventing this tradeoff by artificially xe2x80x9cenlargingxe2x80x9d the effective size of the test material volume by repeatedly transmitting the acoustic signal through same small material volume. Through this new technique, measurements can be performed on material volumes on the order of a wavelength in length that have sensitivity comparable to conventional measurements requiring volumes measured in hundreds or thousands of wavelengths. This allows for the identification of localized, residual material defects as may have been caused by the original stamping or engraving process.
In accordance with an embodiment of the invention, this increased sensitivity is attained in such a small material volume by establishing an oscillating ultrasonic circuit in which the material being measured forms a component in the electro-acoustic oscillator. In this way, the sensitivity of the measurement is no longer controlled by the physical extent of the material volume under test, but rather by the length of time over which the oscillation is monitored. The technique and apparatus for performing such measurement in accordance with the instant invention utilizes a technique similar to scanning acoustic microscopy. However while scanning acoustic microscopy provides high spatial resolution, the measurement sensitivity to variations in the material properties itself suffers because of the relatively short propagation path in the solid material. To gain increased measurement sensitivity to variations in the material, the concept of signal re-transmission is employed. That is, a signal is repeatedly sent through the same small volume of length of the material so that the effects of the material acoustic attenuation and velocity would accumulate as if the signal were propagating through several inches of that material.
A simple means of re-transmitting the received signal in accordance with one embodiment is to apply an amplified version of the output signal from the receiver to the transmitter input. In practice, this circuit results in the establishment of an oscillating circuit. The frequency of this oscillation is determined by (a) the transmit time from the transmitter to the receiver, and hence the acoustic velocity of the solid material, and (b) the system center frequency of the transmit/receive transducers and electronics. In this invention an oscilloscope is used to monitor the oscillation frequency, which, as the signal transmission is repeated N times, will be dominated by a single frequency. Since the frequency of oscillation is dependent upon the acoustic velocity of the solid material, any monitored variation of the frequency of oscillation over time corresponds to a variation in the acoustic velocity of the material itself. In this way, small variations in acoustic velocity of the material may be monitored by utilizing a conventional laboratory-grade oscilloscope display.
Through such a system and method, an embodiment of the instant invention may be employed to recover stamped or engraved serial numbers that have been removed through machining by making sensitive measurements of localized variations in material ultrasonic properties on the specimen surface. These small, residual variations are a result of the stamping or engraving process, and may be detected even after the serial number has been machined away. Such recovery of serial number information is useful for law enforcement, particularly in tracing firearms and weapons that have been involved in a crime. Further, such recovered serial number information may aid in the identification and tracing of auto parts that have been removed from a stolen vehicle.
Other objects and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.